1. Field of the Invention
This invention concerns a method of manufacturing a thick film resistor element. More particularly, this invention concerns a method of manufacturing a thick film resistor element which provides an accurate resistance value.
2. Description of the Prior Art
Conventionally, a thick film resistor element is formed by use of a printing method.
For example, first, an alumina substrate is prepared. Then, a plurality of electrode pairs, e.g., thicker than 10 .mu.m, are formed on the substrate using a thick film printing method. Specifically, a conductive paste layer is applied to the substrate through a screen mask, followed by a drying and a baking treatment.
Next, a plurality of resistive layers are formed using the thick film printing method. These resistance layers correspond to each pair of electrodes so as to partially overlap the electrodes at the end portions thereof.
Then, the substrate is divided into a plurality of chips by scribing techniques so as to produce usable chips.
FIG. 1 is a plan view of a thick film resistor device in which a plurality of thick resistor elements are formed in an array form. For convenience of description, the relationship between the electrodes 2a and 2b and the resistor layers 3 (right column) is illustrated in FIG. 1 as if the resistor layers 3 are transparent, while in actual practice there are not.
FIG. 2 is a cross sectional view taken along the line I--I in FIG. 1. In the drawings, numeral 1 designates an alumina substrate, and 2a, 2b and 2c are electrodes. Numeral 3 designates a resistor element which extends so as to overlap with the electrodes 2a, 2b and 2c partially at the end portions thereof.
In the above described conventional thick resistor elements, the accuracy of the resistance of the resistor elements is poor. Thus, trimming is usually required to achieve a desired resistance value.
The inventors discovered that the deterioration of the resistance value is mainly due to the distortion of the resistor layer. Namely, since the electrode layers 2a, 2b and 2c are relatively thick, e.g., thicker than 10 .mu.m, the shape of the resistor elements is distorted due to the overlap between the resistor layers 3 and the electrode layers. As the chip size becomes smaller and smaller, the influence of the distortion on the resistance value becomes more and more conspicuous.
Thus, the inventors attempted reducing the thickness of the electrode layers. The inventors used a screen mask of less than 5 .mu.m to reduce the thickness of the electrode layers.
However, small notches 4 were observed at the edge of the electrodes layers, as shown in FIG. 1. The notches are formed because of the mesh construction found in the screen mask used (not shown). The influence of the small notches to the resistance value becomes serious when the chip size becomes small, e.g., 1 mm.sup.2. As a result, the accuracy of the resistance value becomes less.